If it has carbonation already, you should be all set. But just to make sure, chill the keg for 24 hours, then attach your CO2 line, turn the pressure up to 25-30 psi, and rock the keg back and forth. Once you stop hearing the CO2 going into the keg, it's good. Then lower the pressure to around 5-10 psi, disconnect the CO2 line, release the pressure in the keg, and then hook everything up, and pour.
I am not a fan of rocking the keg, I did it twice, and overcarbed both times. My process...
1: seat the lid with 30PSI
2: disconnect gas, and chill overnight
3: reconnect gas at 30PSI for 48hours
4: disconnect gas, bleed off pressure in the keg
5: reset regulator to serving pressure (10-12psi for me)
6: reconnect gas, and let sit for 2-7 days
7: get drunk
if I have 3 other kegs being served at the time I add a new keg, I simply connect gas at serving pressure and let it sit for 2-3 weeks
when naturally carbing...
1: add priming sugar and close lid
2: apply 30psi to seat the lid, then purge headspace
3: put in brewcloset for 3 weeks
4: put in keezer for 24-48 hours to chill
5: bleed pressure from keg
6: apply serving pressure for 24 hours
7: get drunk
7 is my favorite number.